Tuning HW90 tuning discoveries

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JayJay

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May 29, 2020
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Washington, United States
Well, I finally bought a HW90. There have been several posts about low power, hard cocking, weight and general tuning questions. I decided to bite the pellet, buy one, and figure out what’s going on. So I ordered a new .25 from AoA. I also ordered a Weihrauch HW90 pressure gauge and a .22 barrel.

The HW90 simply reeks of quality. The stock is gorgeous. I really like the distinctive checkering. I measured the overall gun weight at 4275 grams, or about 9.4 lbs. Funny, Weihrauch’s site says 4 kg, or 8.8 lbs. I didn’t actually measure the cocking effort but it was very high.

Gun overall = 4275 grams, or 9 lbs 7 ounces
Beech stock = 1224 grams, or 2 lb 11.2 oz.
Inertia piston = 48 grams
Front sight = 46 grams
Rear sight = 46 grams
Receiver and barrel assy without inertia piston, sights or stock = 2911 grams

I ran a series of tests, measuring the velocity at different ram bar pressure levels.


  • I used the Weihrauch HW90 pressure gauge.

  • To charge the piston I used a Hill Mark IV pump, and two adapters.

    • A Schrader to 1/8 BSPP adapter. Available from Best Fittings in UK.

    • A quick release to 1/8 BSPP adapter. From AoA.

    • You screw the two adapters together, and they connect the Hill pump and the schrader port.

  • The Hill pump’s gauge is meant for much higher pressure levels and doesn’t have fine resolution for this job. But it’ll get you pretty close to 26 bar in about 10 pumps.

  • You start high, and work your way down to the target pressure.

  • The Weihrauch pressure gauge is ideally suited for the job. You just screw it into the schrader port in the back of the gun. You take a reading by depressing the head. Each time you depress the head, it empties its small internal plenum, and takes a new reading. Doing so drops ram pressure about 1/4 of a bar. It’s easy to creep down to the target level.

  • I found the highest power (21fpe) is about 19 bar, not at 26 bar. Any higher made it harder to cock and dropped power dramatically.

  • For UK levels, 14 bar hits about 12 fpe.

  • I swapped in the .22 barrel and retested. The power vs bar curve appears the same for both .25 and .22. 18 bar is the sweet spot for both.

  • I set it at 18 bar. Cocking effort is now quite decent. I'm getting 800 fps with the H&N FTT .22 14.66 grain pellets. That's 21 foot-lbs.

  • I removed the inertia weight and checked velocity. It didn’t change velocity. Cocking effort and recoil also seemed about the same.

Comments


  • It’s extremely sensitive to bar levels.

  • The 26 bar limit is just an engineering maximum. I think the factory sets them all at 26, and expects people to tune them down themselves.

  • Ideally all HW90’s should be sold with the understanding that they need to be tuned.

  • IMHO, the sweet spot appears to be 18 bar. At that level you get maximum power, and cocking effort is very reasonable.

  • People say that gas-ram magnum springers have harsh recoil. I did these tests without the stock, holding the receiver with gloves. I found recoil to be comfortable, even at 26 bar. The bare gun without a stock, iron sights or inertia piston weighs only 6.4 pounds. It’s got me thinking about fabricating a aluminum stock. It may be possible to reduce weight to 7.5 pounds.

  • I ordered a Vortek piston seal, and may retest later. The inertia piston deserves a closer look.

  • To deburr the inside of the receiver holes, I use a swiveling deburring tool, and put it into a pin vise, see photo. It gives you the extra reach. It's long enough to reach through the hole from the other side of the receiver.

  • I didn't run any accuracy tests.

Conclusion - I love my HW90. I would hope that those discouraged by high cocking effort and low power take a second look.

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Wow, great write up!

Maybe a silly question from a novice on tuning, but how different is the Beeman R9? Would this tuning apply?
Click to expand...

I agree, that write up pretty much touched on all issues concerning the HW90.

In answer to your question, the R9 and HW90 are two completely different animals. The HW90 uses Theoben gas strut technology while the R9 is powered by a standard coil spring. In essence, you could do the same thing by experimenting with different springs, etc., which spring gun enthusiasts do all the time, but it won’t be as easy as pumping in some air and then releasing it.

I bought an HW90 about a year ago from a fellow who had it advertised on a firearms forum, and in its factory configuration it’s loud, difficult to cock and horribly inaccurate. Talk about disappointment!

But then I started doing a little reading and, like the OP, bought a pump and began some tests using different pressures. In my case, I found that 22 bar was the sweet spot for my rifle. 

That a relatively minor bit of fiddling could so completely transform the rifle was amazing. It shoots the H&N 20 grain .25 pellets with great accuracy and plenty of power. The firing cycle is mesmerizingly pleasant, and cocking effort went from viciously hard to remarkably easy.

It makes me wonder how many HW90 rifles are languishing in closets throughout the country because of unhappiness with the factory setting that does little more than inspire contempt in new owners.

Tuned down, the HW90 is GREAT spring gun that every airgun enthusiast should have.






 
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Yes the HW 90 is a great rifle set up properly and is the ONLY adjust gas ram rifle I know of. You have perfect equipment to set up the rifle to shoot at it's best well done and great write up! P.S. Custom stock UK could make a very nice stock on the rifle if you wanted similar to the one seen below and you could also order a threaded barrel from chambers to add a moderator as well. Attached is an example of customized HW 90 showing this sort of the ultimate break barrel in my opinion. 



Custom_HW90K_2.1639209964.jpeg

 
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Wow, great write up!

Maybe a silly question from a novice on tuning, but how different is the Beeman R9? Would this tuning apply?
Click to expand...

The R9 is a completely different gun powered by a coiled spring. It's completely different. Here is a video on teardown for your gun.

https://youtu.be/5KjyQ3eWjrM

From there I would just replace the piston seal and spring/guides with a Vortek kit. Fair warning the R9 has one of Weihrauchs goofiest designs as far as disassembly goes. Those 4 blocks are a bit annoying to knock out the first time through.
 
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Guys, thanks for the feedback. I wrote the post out of frustration at the lack of solid HW90 information. I'm an airgun newbie. 

When the Vortek piston seals arrive, I plan to do the following:

  • Try tuning the HW90 without removing the stock. The schrader port is easily accessible and the adapter seems long enough to clear the stock.
  • Test power levels again. The Vortek seal may change power.
  • Look closer at how the inertia piston affects recoil and power. 
  • Measure the bar level when fully cocked. The gauge goes to 40 bar, and might be able to read it. I suspect that the pressure reading when cocked will be more indicative of the actual power level.

@phillip - Yep, you summed it up perfectly. It blew my mind when dropping the ram pressure dramatically increased power. And improved cocking too! It's interesting that you found the sweet spot at 22 bar. I have read that some HW90's had a different stroke lengths. Or maybe it's due to a different piston seal.

@napairgunner - Wow, that really is the ultimate breakbarrel. I love the stock and just emailed Custom stock UK, asking if they ship to the US. You mentioned a threaded barrel. I looked at Chambers, they list #BRL008, Carbine Barrel .22 - HW90K With 1/2" UNF thread. That's exactly what I was looking for. The standard HW90 500mm FAC barrel is unnecessarily long, and the gun has a sharp report. It needs a moderator. -- Unfortunately, Chambers is out of the threaded K barrels. Do you know if a HW95 barrel will fit? There seem to be conflicting opinions on Weihrauch barrel interchangeability. I have an HW95K 'field pro' from AoA. It's short, with the factory moderator. I was considering pulling the barrel and see if they're the same. 

@CyberDyneSystems The post by Sqwirlfugger57 is 100% correct. I put a Vortek PG3 kit in my HW95k. It boosts power from mid 15 fpe to the high 16's, smooths out the shot cycle, removes twang, and adds just a tiny bit of cocking effort. Definitely worth the upgrade, although the stock HW95 is great. 
 
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I can remember when gas ram rifles first hit the market, they were hard to cock and firing behavoir was rough. Back then they were not adjustable, you had to return them to the factory for all adjustments. While high on power, they were just not very pleasant to shoot. Thank goodness they have improved and we now have the ability to adjust these ourselves....unfortunately many out there do not know or understand that the gas rams of today can be tuned. Thanks for your update and tuning info, hope it brings others around to dust off there older rifles and start shootin !!
 
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@AirShot

I feel very happy that I can contribute something to the airgun community. I was a piano tuner/tech for a number of years. Air gunners and piano tuners have a common mentality, a desire to tune the instrument and make it the best it can be. The HW90 is the epitome of user-tuneability.

Now, if only Weihrauch would come out with a line of mini HW90s.
 
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I can remember when gas ram rifles first hit the market, they were hard to cock and firing behavoir was rough. Back then they were not adjustable, you had to return them to the factory for all adjustments. While high on power, they were just not very pleasant to shoot.
Click to expand...

I remember too. They were adjustable. I bought one. Cocking effort is as high as you want to make it. Mine shot better than my R10. Was more pleasant to shoot than the twangy hold sensitive R10. It also came with full service instructions. Rebuild and trigger adjustment. I bought the factory pump for adjusting ram pressure.with the gun.
 
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@Crowski and KWK

I wish you the all the best! 

Yep, the HW90 is a wonderful gun that got a bad rap. I'm hooked.

Now that I'm comfortable with dropping ram pressure and lowering cocking effort, I'm thinking of putting together a lightweight 90 with a short (K) carbine barrel. The long barrel is heavy, and I don't think you get that much extra velocity. I might try to make a lightweight stock out of pop-riveted aluminum extrusions (from Home Depot). We'll see...

But HW90 K barrels are hard to come by. I wrote Chambers (gunspares.com) in the UK and asked about swapping in a HW95 barrel. They said breech blocks are identical, the only difference is the information engraved on the side. I would assume HW80 barrels work too. That gave me the courage to try my HW95 barrel in my HW90. The barrel block fit and appeared to lock up perfectly. 

By eye, the HW95 one-piece cocking link appeared to be the same exact length and overall shape as the HW90 two piece link. They are the same thickness, so should be equally strong. I did not actually measure the differences. I didn't test swap them to see if the 95 link will work with a 90 receiver. The main unknown is how the 95 link interacts with the 90 shoe. When cocking, the one piece link presses down on the shoe for part of its cycle. The two piece link has a delrin slider in the middle to carry that downward load. This probably explains why the one-piece shoe has slots on its side, to carry the downward force when cocking. So swapping links would probably necessitate swapping shoes as well. The HW90 receiver may not accept the HW95 shoe.

But the HW95 link is dramatically lighter than the HW90 articulated link. I might tinker with that when it gets warmer.

Weihrauch uses special link pins with countersunk ends that you peen over for retention. They're not cheap, and peening distorts the ends so you need an arbor press or very careful hammer and punch to drive them out. With care they can be reused. I ordered a new pin from AoA, it measures 4.99mm (5mm) in diameter, by 18.55mm long. The other hinge pin is the HW90 two piece link middle pin, I drove one out, it's the same exact size. I think they're all 5 x 18 mm. So theoretically you could use 5mm drill rod pieces for the hinged points and locktite them. Or use a #9 US drill, which is also .196". That gives you the benefit of inexpensive hardened steel pins. But you would have to keep an eye on them, in case the loctite failed and the pin worked out sideways. I would use a genuine pin for any permanent work.

Being retired helps.
 
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r1lover,

You're absolutely correct about recoil being heavier with a lighter gun. It's been a long time since my college physics classes! You're also right that it can damage a scope. I now use red dot sights on my springers for that reason. They seem to be impervious to recoil and are a good alternative to regular iron sights, as long as you have fairly good vision. Here's the one I use: https://www.primaryarms.com/primary-arms-slx-md-20-advanced-push-button-20mm-micro-red-dot-sight

I tried shooting my HW90 at 21 ft-lbs without a stock. It really wasn't that bad. Granted I wore leather gloves, because the receiver has some sharp edges.

I don't pretend to be a bench-rest or long range shooter. I'm more interested in gun 'handleability' if there is such a word. 

By the way, I really don't recommend cocking and shooting a HW90 (or any springer) without the stock. Not only are you depriving the receiver of its support structure, you also run the risk of the articulated cocking arm inverting and popping out. That actually happened to me, it whacked my finger, although no damage was done. These things have a super hair trigger, so always mount the trigger guard if you cock the gun.
 
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One last post.

I bought these items from Arizona Airguns.


For the shrader-1/8" BPSS adapter you need to go to Best fittings.


Best Fittings sell complete kits for a reasonable price. My only concern would be the quality of the Slim Jim pump. But you can always pick up an inexpensive manual PCP pump later.

Best Fittings has an EBAY store too, which might simplify your orders. 

 
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@Fkh84

Interesting. If the vibration stopped after installing a dampa mount, then maybe the other mounts were loose. 

21 bar is a fairly stout power setting. The HW90 is not kind on scopes.

I switched from scopes to a Primary Arms MD-20 red dot. Although it weighs only 4 ounces (120 grams), the recoil is so strong that I must use a retaining pin, or it slides right off the dovetail. 

I prefer red-dots over scopes. They save lots of weight, and are almost invulnerable to recoil. 




 
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